Playing the legendary campaign on all 3 characters gives at least 18 powerful drops over those characters plus finishing the Witch Queen campaign you get a full gear set at 1520. I'm assuming that your issue is the same as mine why you're not 1550 yet you don't have the Witch Queen expansion so you're not getting all the drops you could? That being said, I'm a hardcore gamer and I already have my characters over 1540 without the expansion. I don't know how you're still at 1518 unless you only play maybe an hour or 2 a week?? The only thing I can think of is you completed the full campaign before getting your characters to 1500 by doing public events and playlist activities.